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Annulenes, Heterocyclic and Polycyclic Aromatic Compounds

Heterocyclic Aromatic Compounds

Cyclic compounds whose rings do not only contain carbon atoms may be aromatic, as well. In heterocyclic compounds one or more carbon atoms of the ring are displaced by atoms of other elements, which are then called heteroatoms. Heteroatoms in organic compounds are usually nitrogen, oxygen, or sulfur atoms. In aromatic heterocyclic compounds, it is not only the carbon atoms' orbitals that participate in the formatin of the aromatic π system. Orbitals of the heteroatom(s) take part in the π system here too. Pyridine, for example, is such an aromatic heterocyclic compound. Each of the six ring atoms is sp2-hybridized, while the molecule possesses six π electrons. The lone electron pair of pyridine's nitrogen atom occupies an sp2 orbital. These electrons are therefore not π electrons. The remaining two sp2 orbitals of the nitrogen overlap with sp2 orbitals of the adjacent carbons, forming two σ bonds. The π system is the result of the overlapping of the nitrogen's and carbons' p orbitals, which are perpendicular to the ring plane.

In pyridine, the nitrogen's lone electron pair dominates the chemical character, as the negative potential is mainly located at the nitrogen (red region in the illustration). In the case of the heterocycles furan, pyrrole, and thiophene, the aromatic character, as depicted in the illustration, increases in order of "furan" < "pyrrole" < "thiophene", while the diene character decreases, respectively. That is, the region with the highest electron density (red region) is shifted farther away from the heteroatom to the center of the π system the lower the heteroatom's electronegativity is, as the lone electron pair is stronger retained by more electronegative heteroatoms.

Some important heterocyclic compounds


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